PLANT AVAILABILITY OF HEAVY-METALS IN SOILS PREVIOUSLY AMENDED WITH HEAVY APPLICATIONS OF SEWAGE-SLUDGE

Plant uptake is one of the major pathways by which sludge-borne potent ially toxic metals enter the food chain. This study examined the accum ulation of Cd, Cu, Ni, Pb and Zn in wheat, carrots and spinach grown o n soils from 13 sites previously amended with sewage sludge. Winter wh eat, carrots and spinach were grown consecutively under field like con ditions. The results showed that plant availability of heavy metals di ffered widely among the crop species. The accumulation of Cd, Ni and Z n in the plants showed the greatest increases compared to their backgr ound levels. The Cu and Pb accumulation in the plants grown on sludge- amended soils showed only small increases compared to those grown on u ncontaminated soils. Multiple regression analysis of various soil prop erties showed that the surest way to control the accumulation of metal s in food plants is by controlling their concentrations in the soils. Furthermore, soils with a non-acidic pH and a clayey texture tended to achieve better control of metal accumulation in food plants compared to those with an acidic reaction and a coarse texture. Metal concentra tions in the plants generally correlated well with those extracted fro m soils in 0.005 M DTPA, 0.05 M EDTA-(Na)(2), 1 M NH4NO3 and 0.05 M Ca Cl2. The EDTA, however, proved to be a more reliable and consistent te st in predicting the accumulation of metals in the plants. The results also showed that liming soils to pH 7 effectively reduced the metal c ontents in carrots and spinach, but liming to pH 6.5 had little effect on metal concentrations in wheat grain.